Effect of strong Borrmann absorption on X-ray fluorescence yield curves

The Ga K X-ray fluorescence yield from GaAs was measured around the 200 reflection peak in the symmetric Laue case, by tuning the incident X-ray energy from a synchrotron radiation source between the K-absorption edges of Ga and As. It was found that the fluorescence yields from the crystal surface become minimum at the exact Bragg angle, when the diffraction is induced only by χhi, i.e., the imaginary part of the Fourier coefficient of the X-ray polarizability (χhr being the real part). In the case of |χhr| = |χhi|, which occurs at two energy points, both yield curves are asymmetrical with respect to the exact Bragg angle and their asymmetries are opposite to each other. (author)